Carboxyl-terminated linear polyester gas-generating composition and method of preparaion



United States Patent ()filice 3,1711% Patented Apr. 6, 1965 3,177,101CARBUXYL-TEATED LINEAR PGLYESTER GAS-GENERATING CQMPGSITKQN AND METH- DOi PREPARATIQN Calvin W. Vn'esen, Erookside, Del., assignor to ThiokolChemical Corporation, Bristol, Pa., a corporation of Delaware NoDrawing. Filed July 2, 1962, Ser. No. 207,963 9 Claims. (Cl. 149-19)This invention relates to gas-generating compositions of the type usedin auxiliary power sources to generate gases 7 under pressure foroperating a variety of pressure fluid actuated devices.

In the aeronautical and astronautical fields lightweight, gas-generatingauxiliary power sources are extensively used for a wide variety ofapplications, particularly those applications wherein a rathersubstantial amount of power of relatively shortv duration is required toactuate some mechanical device. For example, such gas generators areused as starters for jet engines, hydraulic pumps and alternators inelectrical power systems, as well as starters for the large turbo-pumpsystems of liquid propellant rockets. They are also used as a powersource for small reciprocating piston pumps for hydraulic systems. Insome cases the generated gas under pressure is applied directly to aliquid to be moved as for example in the pressurization of the fueltanks of a liquid propellant rocket system.

While the compositions used in such gas generators are functionally akinto the materials used as ballistic and rocket propellants, they differin important respects from the compositions commonly used aspropellants. Thus gas-generating compositions have a substantially lowerburning rate than conventional propellants, just as propellants have asubstantially lower burning rate than explosives. .Also gas-generatingcompositions should have relatively low flame temperatures and should beessentially nonerosive and noncorrosive to prevent damage to valves,turbine blades and the like with which they may come in contact. Forsuch applications as jet engine starters, it is important that thecomposition burn substantially completely to produce an essentiallysmoke-free gaseous product.

Like conventional rocket propellants, gas-generating compositions mustremain ballistically stable over a relatively Wide temperature range andmust be shock-resistant at both high and low temperatures. Also theymust be ignitable at temperatures as low as 70 to 75 F. In general, bothtypes of compositions should have substantial elasticity to provide thedesired stability during storage and use.

Like conventional rocket propellants, gas-generating compositionsusually are essentially composed of a major portion of inorganicoxidizer and minor portion of organic fuel binder. However, ingas-generating compositions it is customary to use ammonium nitrate asthe oxidizer rather than a more energetic oxidizer such as ammoniumperchlorate in order to achieve the desired greater stability in storageand low flame temperature during combustion. Also Whereas in aconventional rocket propellant the oxygen content of the fuel binder iskept to a minimum in order to maximize the energy developed duringcombustion of the propellant, binders employed in gas-generatingcompositions usually contain substantial amounts of oxygen to reduce theflame temperature and to insure substantially smoke-free combustion.

It has previously been suggested that polyester resins be used as fuelbinders in gas-generating compositions. Such polyester resins have longbeen used for general purpose molding applications. They are formed bycondensing pclyhydric alcohols with unsaturated polybasic acids to formlinear polyesters having unsaturated group-s therein. The unsaturatedgroups of the polyesters are then reacted with a compound havingvinyllic unsaturation, e.g., styrene, in the presence of a peroxidecatalyst to form a solid res'm. In making gas-generating compositions,the liquid unsaturated polyester has been mixed with the oxidizer, aquantity of styrene, a peroxide catalyst, and a plasticizer. The vinylgroup of the styrene reacts with the unsaturated groups of the polyesterto produce a solid binder. Compositions of this type are disclosed forexample in Marti Patent No. 3,013,871. As shown in the Marti patent,substantial quantities of plasticizer must be employed in suchcompositions, since the polyester resin is a relatively rigid materialand in the absence of plasticizer does not yield the desired physicalproperties.

The use of a plasticized polyester resin a the fuel binder of agas-generating composition is rather unsatisfactory because of thetendency of the plasticizer to bleed out of the resin binder. Hencethere is a need in the gas generator art for a fuel-binder whichcontains the necessary amount of oxygen to insure complete combustionand which will provide the desired physical properties without theaddition of a separate plasticizer. It is an object of the. presentinvention to provide a gasgenerating composition incorporating such afuel-binder and a method of making such a gas generator composition. Thepresent invention is predicated on the finding that an improved gasgenerator composition can be made by employing as the fuel binderthereof a carboxyl-terminated linear polyester that is cured through thecarboxyl terminal thereof. In accordance with the method of the presentinvention, a liquid polyester having a molecular weight of 500 to 5,000is prepared by condensation of a polycarboxylic acid and a polyhydricalcohol to produce a liquid carboxyl-termina-ted copolymer. The liquidcopolymer is mixed with ammonium nitrate powder and a curing agent forthe copoly-mer selected from the group consisting of polyimines andpolyepoxides. Upon heating to a moderately elevated temperature, say toF., the polyester cures to a solid binder. The resulting compositionsupon ignition burn relatively slowly and with a low flame temperature togive essentially smoke-free combustion products. The cured polyestershave substantial elasticity and thus confer good shockresistance and lowtemperature properties on thegas generator composition, even though noseparate plasticizer is incorporated therein. I 1

Polyhydric alcohols that can be'use d in preparing the linear polyestersinclude ethylene glycol, diethylene glycol, propylene glycol,polypropylene lycol, butanediol,

polybutylene glycols, glycerol, trimethylolpropane, 1,2,6- hexane triol,castor oil and mixtures of these polyols. Suitable polybasic acids forreacting with the foregoing alcohols to form the linear polyestersinclude oxalic, adipic, sebac-ic, phthalic, isophth-alic, terephthalic,malonic, succinic, maleic, furnariqdimer and itaconic acids and mixturesof these acids. The alcohols and acids having more than two functionalgroups can be used in varying amounts in the reaction mixture to producevarying amounts of cross-linking. Polyesters prepared by reacting amolar excess of adipic acid with diethylene glycol to form acarboxyl-terminated linear polyester have been found especiallysatisfactory. Since the linear polyesters are well known in the art,it-is deemed unnecessary to describe in detail methods for making them.

In general, the present compositions are prepared by mixing ammoniumnitrate in powder form with the carboxyl-terminated polyester and asuitable curing agent therefor to form a fluid composition that can becast at atmospheric temperature in a mold of the desired configuration,and then heating to a temperature of 100 to 150 F. to cure the polyesterpolymer to form a solid grain of the gas generator composition. Whileany of the curing agents previously employed for the curing ofcarboxyl-terminated polyesters may be used, good results have beenobtained with compounds having two or more heterocyclic groups that arereactive with carboxyl groups, e.g., polyepoxides and polyimines. Thediglycidyl ether of bis-phenol A sold under the name Epirez 510 istypical of the commercial polyepoxides that may be used, and thealkyleneimine tris[1-(2-methyl) aziridinyl]phosphine oxide sold underthe trade name MAPO is typical of the commercial imine curing agentsthat can be employed in making the present compounds. Other curingagents include the diglycidyl ether of hisphenol F [diglycidyl ether ofbis-(4-hydroxy phenyl) methane]; tris [I-(Z-methyl) aziridinyl]phosphine sulfide; N,N' hexamethylene-bis-l-aziridinecarboxamide;1,3,5-[fi-(2-methyl).aziridinyl]propionoxy pentane; 1,2- [6 (2-methyl)aziridinyflpropionoxy ethane; N,N'- (methylphenylene)bis-l-aziridinecarboxamine; 1,1'-(sulfonyldiethylene)bis-2-methylaziridine; and 1,1'-terephthaloyl-bis-aziridine. Curing iseifected by reaction of the heterocyclic groups of these curing agentswith the carboxyl terminals of the polyester.

As indicated by the specific examples given below, the presentcompositions may contain various minor constituents in addition to theammonium nitrate, polyester and curing agent. Thus the reaction mixtureprior to curing may contain any of various known curing catalysts suchas for example 4,4'- methylene bis(2-chloroaniline); 4,4 methylenedianiline; m-phenylene diamine; diamino toluenes; triethylene.tetramine; tetraethylene pentamine; diethylene triamine;tri(dimethylaminomethyl) phenol and mixtures of these amines. Also ithas been found desirable to use a minor proportion of a combustioncatalyst to improve the reliability of ignition of the composition andto insure continued combustion after ignition. It has been found that asmall amount of b-arbituric acid in the composition is especially usefulfor this purpose.

In preparing the present compositions, a relatively large proportion ofthe inorganic oxidizer, in this case ammonium nitrate, should beemployed. More particularly, the cured composition desirably containsfrom 50% to 80% by weight of ammonium nitrate powder and 20% to 50% byWeight of the cured binder and minor constituents. In the fluid mixtureprior to curing, the liquid polyester component may comprise from 15% to45% by weight and the curing agent from, say 3% to 10% by weight. Thecombustion catalyst, e.g., barbituric acid, may comprise from 0.5% to 5%by weight of the mixture.

In order to point out more fully the nature of the pres ent invention,the following specific examples are given of procedures for making thepresent compositions, and certain of the properties of the curedcompositions are set forth.

Example 1 The polyester used in this example was prepared by condensing9.4 mols of adipic acid with 8.94 mols of diethylene glycol. Theresulting polyester polymer had an acid number of 60 and a viscosity of90 poises at 80 F.

A vertical planetary mixer was charged with 17.35 parts of thispolyester, 8.65 parts of the diglycidyl ether of bisphenol A (Epi-rez510), 1 part of 4,4 methylene dianiline, and 1 part of barbituric acid.These ingredients were mixed for minutes. Thereafter 43.2 parts ofammonium nitrate (3600 r.p.m. grind) and 28.8 parts of ammonium nitrate(16,800 r.p.m. grind) were added, and mixing continued for an additionalperiod of one hour.

One portion of this mixture was cured at F. for 96 hours and then sawedand stamped into dumbbellshaped specimens. Thereafter the specimens weresubjected to a tensile test and exhibited a tensile strength of 33p.s.i. and an elongation of 42%. The modulus of the material was 112p.s.i.

Another portion of the mixture was cured in a gas generator motor, andthe burning rate after cure was determined to be 0.054 in./sec. at 1000p.s.i.a. with a burning rate exponent of 0.45.

The density of a sample of the cured material was determined to be 0.056lb./in.

Example 2 The procedure of Example 1 was followed except that theproportions were changed as indicated below:

Ammonium nitrate (3600 r.p.m. grind) 45.00 Ammonium nitrate (16,800r.p.m. grind) 30.00 Polyester prepolymer 15.86 Polyepoxide 7.14 4,4methylene dianiline 1.00 Barbituric acid 1.00

Samples of this formulation were cured as in Example 1 and exhibited thefollowing tensile properties: tensile strength-65 p.s.i.,e1ong-ation-16%, modulus 580 p.s.i. The burning rate was determined byburning strands of the cured material in a Crawford Strand Burner andwas found to be 0.04 in./sec. at 1000 p.s.i.a. with a burning rateexponent of 0.59.

Example 3 The procedure of Example 1 was followed except that the samplewas cured 40 hours at 150 F. and the proportions of ingredients werechanged as indicated below:

Ammonium nitrate (3600 r.p.m. grind) 42.60 Ammonium nitrate (16800r.p.m. grind) 28.40 Polyester prepolymer 20.80 MAPO 4.02N,N'-hexamethylene-bis-1-aziridine-carboxamide 2.18 Barbituric acid 2.00

Tensile results obtained with cured samples were: tensile strength-46p.s.i., elongation23%, modulus129 p.s.i. The burning rate, determined onmaterial cast and cured in a generator, was found to be 0.04 in./sec.,at 1000 p.s.i.a., and the burning rate exponent was found to be 0.55.

A flame temperature of 1975 F. was measured during the burning test bymeans of a thermocouple.

Example 4 The procedure of Example 1 was followed except that sampleswere cured for 40 hours at 150 F. and the proportions of ingredientswere changed as indicated below:

Ammonium nitrate (3600 r.p.m. grind) 31.95 Ammonium nitrate (16800r.p.m. grind) 39.05 Polyester prepolymer 22.94 MAPO 1.631,1'-(su1fonyldiethylene)bis-2-methyl-azi1idine 2.43 Barbituric acid2.00

Tensile properties of cured specimens were found to be: tensilestrength-61 p.s.i., elongation4l%, modulus-220 p.s.i. The burning rate,determined on material cast and cured in a generator, was found to be0.035 in./sec. at 1000 p.s.i.a. with an exponent of 0.55.

It is of course to be understood that the foregoing examples areintended to be illustrative only and that numerous changes can be madein the ingredients, proportions and conditions set forth therein withoutdeparting from the spirit of the present invention as defined in theannexed claims.

I claim:

1. A gas-generating composition essentially composed of ammonium nitratepowder and a cured carboxylterminated essentially linear polyesterbinder, said linear polyester prior to curing having a molecular weightof 500 to 5000 and being the condensation product of a polycarboxylicacid and a polyhydric alcohol.

2. A gas-generating composition essentially composed of ammonium nitratepowder and a cured carboxyl-terminated essentially linear polyesterbinder, said polyester prior to curing having a molecular weight of 500to 5000 and being the condensation product of a polycarboxylic acid anda polyhydric alcohol, said polyester being cured by the reaction of thecarboxyl groups thereof with a curing agent selected from the groupconsisting of of 50% to 80% by weight of ammonium nitrate powder and 20%to 50% by Weight of a cured carboxyl-terminated essentially linearpolyester binder, said linear polyester prior to curing having amolecular weight of 500 to 5000 and being the condensation product of apolycarboxylic acid and a polyhydric alcohol.

7. A gas-generating composition essentially composed of from 50% to 80%by Weight of ammonium nitrate powder, from 0.5% to 5% by weight ofbarbituric acid as a combustion catalyst, .and a binder which is a curedcarboxyl-terrninated essentially linear polyester, said linear polyesterprior to curing having a molecular weight of 500 to 5000 and being thecondensation product of a polycarboxylic acid and a polyhydric alcohol.8. The method of making a gas-generating composition which comprisesmixing from to by weight of ammonium nitrate powder with 15% to 45% byweight of a carboxyl-terminated essentially linear polyester having amolecular weight of 500 to 5000 and 3% to 10% by weight of a curingagent for curing said polyester through the carboxyl groups thereof,casting the resulting mixture into a desired configuration, and curingsaid mixture to cause the polyester and curing agent to react to form anelastomer.

9. A method according to claim 8 and wherein said curing agent isselected from the group consisting of polyepoxides and polyimines.

References Cited in the file of this patent UNITED STATES PATENTS3,000,714 Batchelder et al Sept. 19, 1961 3,000,715 Lawrence Sept. 19,1961 3,031,288 Roberts Apr. 24, 1962 OTHER REFERENCES Chem. and Eng.News, August 1, 1960, p. 35. Chem. and Eng. News, August 8, 1960, p. 53.

1. A GAS-GENERATING COMPOSITION ESSENTIALLY COMPOSED OF AMMONIUM NITRATEPOWDER AND A CURED CARBOXYLTERMINTATED ESSENTIALLY LINEAR POLYESTERBINDER, SAID LINEAR POLYESTER PRIOR TO CURING HAVING A MOLECULAR WEIGHTOF 500 TO 5000 AND BEING THE CONDENSATION PRODUCT OF A POLYCARBOXYLICACID AND A POLYHYDIRC ALCOHOL.